Hyperpolarization-activated cyclic nucleotide-modulated (HCN) ion channels were first discovered in photoreceptors where they shape the light response. They exhibit several properties that make them specialized for retinal signaling: 1) they are activated by membrane hyperpolarization instead of depolarization, 2) they are regulated by the direct binding of cyclic nucleotides to an intracellular domain, and 3) they are expressed in the distal dendrites of neurons. Recently an accessory subunit of HCN channels in photoreceptors and other neurons was discovered called TRIP8b that has a profound effect on each of these important channel properties. Our long term goal is to understand the molecular mechanisms for these properties. In previous funding periods we have made great progress toward achieving this goal. We have solved the X-ray crystal structure of the cyclic nucleotide-binding domain of HCN2 and the structure of TRIP8b bound to HCN2. We have also invented three ground-breaking new fluorescence methods that allow us to record molecular rearrangements in intact channels simultaneous with electrophysiological recording. In this funding period, we propose to combine these methods with double electron-electron resonance (DEER), a powerful magnetic resonance-based method, and molecular dynamics simulations, to measure and model the structure and dynamics of the HCN channel and its interaction with TRIP8b. These experiments will lead to the first dynamic picture for how HCN channels regulate the excitability of photoreceptors and other neurons.